In the last few decades, space-based laser communications (lasercom) has emerged as a transformative technology for scientific, defense, and commercial spacecraft applications. Compared with traditional radio frequency (RF) communications, lasercom offers higher bandwidth, reduced size and mass of transceivers, and lower power consumption. Lasercom also avoids the significant regulatory hurdles of radio frequency allocation and provides better link security with narrow beams. As spacecraft generate increasing amounts of data, space-to-ground lasercom can provide high rate downlink capability to overcome a communications bottleneck.
Small satellites in particular have great potential to benefit from lasercom as they are significantly constrained in size, weight, and power. The small satellite market is currently experiencing a period of rapid growth. Despite launch delays, thousands of small satellites are expected to launch in the next five years.
FIG. 1 shows actual and project small satellite launches from 2010 through 2022. Despite launch delays, the market continues on an upward trend.
As the number of satellites in orbit grows and their capabilities improve, the amount of data they generate puts pressure on existing RF communications infrastructure, particularly as low latency downlink is a priority for many satellite operators. The communications subsystem continues to be a limitation for small satellites. The time required to license a portion of the RF spectrum often takes longer than the entire time to design, build, and test the satellite. The amateur band is becoming overcrowded with small satellites and licensing organizations such as the Federal Communications Commission are straining to keep up with increased demand.
The shortwave infrared spectrum utilized in lasercom has few regulations. Unlike RF, the lasercom spectrum does not require official allocation due to its narrow beamwidths that present little risk of interference. As of this writing, the only restrictions for lasercom frequencies are focused on eye safety and interference. The American National Standards Institute provides a metric for the maximum permissible exposure (MPE), which limits the power flux (in W/cm2) of the signal and is dependent on wavelength. For lasercom downlinks, transmitted power is spread out over a large area and does not approach the MPE limit. Lasers directed upwards are controlled by the Department of Defense Laser Clearinghouse (LCH), which provides guidelines on reporting laser use to protect DOD assets. Safety limits must be considered on uplink since the power is concentrated on the ground. Careful design and precautionary measures such as airplane spotters can avoid safety concerns.
Lasercom Ground Stations for Satellite Communications
Existing lasercom ground stations are limited in number and are typically built into locations with existing infrastructure, such as astronomical observatories. A major challenge for lasercom ground stations are the limitations imposed by weather. A cloud-free line of sight between the space and ground terminals is required, which reduces the availability of individual sites. Diversity in the locations of ground stations is essential for link availability.
The initial investment cost can be high for optical ground stations, ranging from $1-5 million for a low-Earth orbit (LEO) to ground link. The initial investment cost makes development of a diverse ground network challenging. While pay-by-the-minute ground networks exist over RF, no similar option exists for lasercom. RF antennas can be purchased off-the-shelf, but the precision pointing required of optical ground stations requires a more custom solution.
Telescope and Site Selection
A fundamental design parameter of the ground station is the telescope diameter. A larger telescope collects more photons, but the desirability of a large aperture also depends on modulation scheme. The most common scheme is intensity modulation, which carries information based on the presence or absence of photons at a given time. However, some lasercom missions utilize coherent communication, in which case the phase of the incoming signal must also be recovered.
In the case of incoherent (i.e., direct) detection, the diameter of the telescope is selected based on the required power at the receiver. The diameter of the telescope goes into the link budget along with other parameters including transmitter power, path length, and beam divergence. It may be advantageous to split the collection area into several smaller telescopes, which can avoid the challenges of constructing a single large telescope. However, this presents its own challenges regarding alignment and the coupling of signal among multiple telescopes. Regardless of the specific implementation, a design driver of these systems is to provide enough aperture to meet a desired link margin.
The desired aperture size of a coherent ground station is somewhat more complicated due to atmospheric effects. In this case, there are two possible paths based on the Fried parameter, r0. The Fried parameter has units of length and is defined as the diameter of a circular area over which the RMS wavefront aberration after passing through the atmosphere is 1 radian. For practical purposes, the Fried parameter is the diameter at which a diffraction-limited telescope has approximately the same resolution with or without turbulence. For a telescope with a diameter D where D/r0>>1, the atmosphere causes significant spreading of the signal in the focal plane. A telescope with D/r0<<1 will be diffraction-limited based on the aperture size.
The Fried parameter depends on location and time of day, but can be on the order of 10 cm at an astronomical observatory under favorable conditions at a wavelength of 500 nm. For coherent ground stations, either the aperture should be reduced below the Fried parameter or adaptive optics should be used to compensate for atmospheric turbulence. Constraining the aperture presents challenges in the link budget, so generally adaptive optics are preferable despite the additional complexity.
The location of the ground station is driven primarily by weather. Optical ground stations have many characteristics in common with astronomical observatories and are sometimes co-located. Desirable characteristics include high altitude, low humidity, few clouds, and low-strength turbulence. An additional consideration is spacecraft visibility, which could be a limitation for locations at extreme latitudes.
Pointing, Acquisition, and Tracking
Lasercom systems align the optical line of sight very precisely. Existing systems have error as small as submicroradians. For a ground station, this can be accomplished by steering the telescope alone and using a fine pointing stage. The signal is initially acquired with coarse sensors and open-loop pointing of the telescope. The stages of control are coordinated for handoff such that the coarse stage can maintain the signal within the range of the fine stage.
Prior to tracking a satellite, the ground station is aligned and calibrated. A pointing model accounts for the orientation of the telescope/mount assembly in inertial coordinates. This model can also account for mechanical properties of the telescope and common sources of error. The pointing model is generated by taking observations of known celestial objects.
FIG. 2 shows a process 200 of pointing, acquisition, and tracking for a typical lasercom terminal. The process 200 begins with open-loop initial pointing of the ground station telescope towards the satellite and/or vice versa (step 202), followed by open-loop coarse scan and detection of the satellite by the ground station and/or vice versa (step 204). In response to successful detection of the satellite, the ground and space terminals switch to closed-loop fine sensing and pointing (step 206). Once the ground and space terminals have locked onto each other, communications can begin, and the ground and space terminals continue with closed-loop fine tracking and link maintenance (step 208) until communications end.
In the process 200 of FIG. 2, the ground (space) terminal is closed-loop tracking the signal from the opposing space (ground) terminal. For satellite-to-ground communications, the initial satellite pointing error is usually larger than the downlink beamwidth. The ground station and spacecraft go through an acquisition procedure to make sure each terminal can see the other.
The majority of space lasercom terminals use beacon tracking to locate the ground station. In this approach, the ground station sends up a wide beam towards the spacecraft. The spacecraft sees this signal and corrects its pointing error so that the ground station is illuminated with the downlink. While beaconless tracking reduces system complexity and is particularly appealing for deep space lasercom, it is challenging to implement in practice. Without a beacon, the satellite must point within the accuracy of the downlink beamwidth without receiving any feedback from the ground station. Regardless of the use of a beacon uplink, the ground station typically closed-loop tracks the downlink.
FIGS. 3A-3C show the pointing, acquisition, and tracking sequence for a lasercom link between a ground station 310 and a satellite 320. From the perspective of the ground station 310, the sequence begins with initial pointing of a beacon 312 towards the expected location of the spacecraft 320 as shown in FIG. 3A. Orbital knowledge usually comes from GPS, radio ranging, or two-line element sets (TLEs) published by the Joint Space Operations Center (JSpOC), and any error in the spacecraft's position is translated into pointing error. The mispointing induced by position error gets worse the closer the spacecraft is to the ground station.
The ground station then enters the acquisition sequence (FIG. 3B). The ground station 310 open-loop tracks the expected trajectory of the spacecraft 320 and can perform a scan until a signal is seen. Coarse sensors with a wide field of view (FOV) 322, such as cameras, are used in the acquisition phase. Once the downlink signal has been detected, the ground station 310 can close its tracking loop and transition to using its fine pointing stage. When the ground station is fine tracking onto its receiver, the satellite 320 can transmits an optical signal beam 324 as shown in FIG. 3C.
For bidirectional systems, the final step is link maintenance which is conducted throughout communications. The purpose of link maintenance is to ensure transmit/receive (Tx/Rx) path alignment. This can be done by slightly adjusting pointing angle and applying a correction based on received power at the opposing terminal. Alternatively, the system can be designed with self-test capabilities through the use of optical elements to redirect transmitted signal into the receive path for alignment.
Instrumentation
In operation, the ground station collects signal onto the receiver. The design of the ground station optical assembly may be mission-specific, but there are key components that ground stations have in common. These generally include an acquisition sensor, a fine pointing sensor, a fine pointing actuator, a beacon, and a receiver. In the case of coherent lasercom, a wavefront sensor and corrective actuator may also be used.
The choice of receiver architecture drives the design of the optical assembly. The three general architectures are direct detection without preamplification, direct detection with preamplification, and coherent detection. The most common receiver choice is direct detection with a photodiode. Avalanche photodiodes (APDs) can have sensitivities in the photon-counting regime. High photodiode bandwidth typically requires low capacitance, which in turn limits the maximum detector area. This presents a challenge for the ground station to collect signal onto a receiver with a diameter of 200 microns or less. The size of the APD is a design trade for the system.
Coherent receivers necessitate adaptive optical elements as well as a local oscillator. For direct detection, optical preamplifiers, such as those in terrestrial fiber systems, can greatly improve the sensitivity of receivers. However, this involves coupling of the signal into a single-mode fiber with a diameter of around 9 microns. This again calls for adaptive optics to correct spreading of the signal due to atmospheric turbulence.
Apart from the receiver, the ground station instrumentation includes sensors and actuators to assist with pointing, acquisition, and tracking. Some ground stations have multiple levels of coarse sensors and actuators to achieve the desired pointing accuracy. The acquisition sensor is commonly a focal plane array or a quadrant detector with a wide FOV. This sensor is used to spot the signal and steer it into the FOV of the fine sensor. The fine sensor is also usually a focal plane array or quad detector with a narrow FOV. A fast-steering mirror (FSM) is most commonly used for precise tracking. An FSM is a flat tip/tilt mirror with bandwith in the hundreds or thousands of Hertz. An alternative approach uses a nutation element on the receive fiber paired with a photodiode to measure received power and adjust the fiber line of sight.
Challenges for Optical Ground Stations
Atmospheric Effects
The twinkling of stars at night is a well-known example of how light is distorted as it propagates through the atmosphere. To establish an optical link, a clear line-of-sight is desired between both terminals. The severity of atmospheric effects ranges from complete link unavailability, as in the case of cloud cover, to milder effects that can be addressed in the design of the terminal.
To ensure link availability, a diverse ground station network can overcome issues of cloud cover. At a given time, single-site availability may range from 60-80% while three or more sites can achieve over 90% availability. Transportable ground stations can mitigate the effects of seasons and enable dynamically deployed networks.
Beyond cloud cover, the lasercom terminal should be designed to handle general atmospheric conditions. Three atmospheric effects on beam propagation are absorption, scattering, and refractive-index fluctuations. Absorption and scattering tend to cause signal attenuation, while refractive-index fluctuations (i.e., turbulence) tend to cause irradiance fluctuation and loss of spatial coherence.
The atmosphere has different effects on downlinks and uplinks. In downlinks, the atmosphere is close to the receiver, whereas in uplinks the signal passes through the atmosphere immediately upon transmission. By the time the downlink reaches the atmosphere, it has spread out, so it tends to experience scintillation and changes in angle-of-arrival. The downlink scintillation benefits from an aperture-averaging effect. The uplink experiences scintillation and a more severe beam wander. Since the beam has not spread out significantly as in the case of the downlink, the entire signal can be deflected, typically on the order of a few microradians. These atmospheric effects must be carefully considered in the link budget and the design of the pointing and tracking system.
Pointing, Acquisition and Tracking of Low Earth Orbit (LEO) Satellites
A particular challenge for optical ground stations is tracking LEO satellites. Ground station passes last less than 10 minutes, so the ground station must quickly acquire the downlink to maximize data transmission.
A common mount type for LEO tracking applications is the altazimuth mount. An altazimuth mount has one gimbal that provides 360-degree motion in azimuth and a second gimbal that provides 90-degree rotation in altitude (elevation). When trying to track through zenith with an altazimuth mount, a singularity occurs: at zenith, the azimuth should instantaneously rotate through 180 degrees. The result is that any satellite pass that approaches zenith produces very high slew rates in the azimuth axis. FIG. 4 shows the maximum azimuth rate as a function of satellite altitude and elevation. The lower the satellite altitude and the higher the elevation, the more challenging it is for the ground station to track the satellite.
This problem occurs in any two-axis mount. Equatorial mounts, which are a common choice for astronomical observatories, simply tilt this singularity to be aligned with the Earth's polar axis. Unfortunately for altazimuth mounts, zenith passes also correspond with the shortest link range and therefore the most favorable link conditions. Although high elevation passes are rare, the mount should be capable of fast slews to ensure the link can be maintained near the singularity.
Transportability
The transportability of an optical ground station presents several unique challenges as compared to fixed optical ground stations. It should be designed to be compact and easily stowed for transport. This is particularly taxing on the optical assembly. The sensors, actuators, and optical elements should be coupled into the optical path of the telescope. There are two approaches to this. The preferred but more costly approach is to implement Coudé path in which the signal of the telescope is directed to a fixed location below the assembly. A controlled laboratory space with an optical bench can be set up below the telescope. This is a common approach for fixed optical ground stations, but is not well-suited for transportability.
Instead, the optical assembly can be mounted directly to the telescope. Whereas a Coudé path provides as much space as desired for the optical assembly, weight and volume are important constraints for a transportable ground station. The optical assembly should have a small size so as not to imbalance the telescope, and it is desirable to minimize the number of optical elements.
Another challenge for transportable ground stations is the initial calibration and development of a pointing model. For fixed ground stations, the telescope calibration is done by sighting dozens of individual stars. This process is time-consuming but is very stable once it is complete. For transportable ground stations, the calibration should be conducted whenever the telescope is moved. It is therefore desirable to have this process be as rapid as possible.
Existing Optical Ground Stations
Fixed Existing Optical Ground Stations
This section focuses on two examples of fixed optical ground stations that have supported high-profile lasercom demonstrations. These are the European Space Agency's Optical Ground Station (ESA-OGS) and NASA JPL's Optical Communications Telescope Laboratory (OCTL).
FIG. 5 shows ESA-OGS, which is located at the Teide Observatory on Tenerife Island, Spain. ESA-OGS was constructed to support the Semiconductor-laser Inter-satellite Link Experiment (SILEX). Space-to-ground communication between the telecommunications satellite ARTEMIS and ESA-OGS was first established in 2001. ESA-OGS has been used extensively in support of lasercom missions including the Laser Utilizing Communications Equipment (LUCE) mission by the Japan Aerospace Exploration Agency (JAXA) launched in 2005 and the French Liaison Optique Laser Aéroportée (LOLA) aircraft-to-space link demonstrated in 2006. ESA-OGS was one of several optical ground stations utilized in the Lunar Laser Communications Demonstration (LLCD) in 2013, which established lasercom links between a lunar orbiter and Earth.
While ESA-OGS originally only supported lasercom links based on intensity modulation, such as on-off keying (OOK), it has been retrofitted with adaptive optics to correct atmospheric phase distortion. This allows for higher data rates using homodyne binary phase shift keying (BPSK) modulation, which is used by the next-generation lasercom terminals on the European Data Relay System (EDRS) currently being implemented.
FIG. 6 shows the full ESA-OGS telescope assembly. It has a 1-meter aperture in a Ritchey-Chrétien/Coudé configuration. The telescope is attached to an English equatorial mount. The telescope has a focal ratio of f/39, which corresponds with a field of view (FOV) of 2.3 mrad. The Coudé path leads to an enclosed laboratory below the telescope where a 5×2 m2 optical bench contains the receive and transmit optics.
On the receive path, the light from the telescope is collimated and directed onto an FSM that provides steering corrections. A portion of the received signal can be directed onto a wavefront sensor. The remainder of the signal goes through a beamsplitter with a pass-through aperture at its center. This aperture is the size of the tracking sensor's FOV, so that if the signal is within the FOV it passes through; otherwise, the signal is directed to an acquisition sensor with a wider FOV. The signal passing through the beamsplitter is directed through an optical isolator for the transmit and receive beams. A final beamsplitter divides the signal between the tracking sensor and the receiver/analysis equipment.
The transmitter is a titanium-sapphire laser capable of providing 7 W output power at 847 nm, pumped by an argon laser. A second steering mirror is used for aligning the transmitted and received signals and implementing a point-ahead angle as needed. The transmitted signal goes through the optical isolator and couples into the path that the received signal follows.
During a communications pass, the telescope tracks the satellite position from known orbital information in an open-loop manner (blind pointing). The satellite scans a beacon over its cone of pointing uncertainty, and the ground station waits to see a signal on its acquisition sensor. Once the signal is seen, the fine pointing mirror steers the received signal onto the tracking sensor and the ground station initiates its beacon to point towards the satellite. If the fine pointing mirror approaches the edge of its range, it can offload to the telescope mount. When the ground station has locked on to the received signal, it maintains the signal within four pixels on a tracking sensor which acts as a quadrant detector. At this point, the ground station is ready to receive data.
FIG. 7 shows the NASA JPL Optical Communications Telescope Laboratory (OCTL), which was built starting in 1999 to support to support space-to-ground lasercom missions. OCTL is located at the Table Mountain Facility in the San Bernardino Mountains in southern California. OCTL conducted a 50 Mb/s space-to-ground link with JAXA's LUCE terminal in low Earth orbit (LEO) in 2009. It was used as one of the ground stations for LLCD to support lunar downlink rates up to 78 Mb/s. OCTL was also used for the Optical PAyload for Lasercomm Science (OPALS) mission which completed downlinks from the International Space Station (ISS) in 2014.
OCTL has a 1-meter telescope that is in a Coudé configuration and is attached to a high-speed altazimuth mount. The focal ratio of the telescope is f/76, corresponding with a diffraction-limited FOV of 500 μrad. Co-aligned with the main aperture is a 20 cm, f/7.5 Newtonian telescope used for acquisition. This telescope has a FOV of 5 mrad and is aligned to well within the FOV of the main telescope, ensuring a signal can be guided into the main telescope. Additionally, a CCD camera with a motorized zoom lens provides a coarse FOV of 34 degrees down to 1.7 degrees for use in coarse acquisition. Initial acquisition can be achieved with the wide FOV camera or the acquisition telescope, which provides feedback to bring the target within the FOV of the main telescope.
A mirror along the Coudé path can direct the signal to any of four different optical benches so that ongoing missions can coordinate time on the telescope. The optical assembly depends on the specific mission, but an FSM is generally used for fine pointing. For LLCD and OPALS, a monostatic configuration was used in which the transmitted signal goes out of the main telescope. The mount can slew up to 20 degrees/s in azimuth and 10 degrees/s in elevation, which enables tracking of LEO satellites.
Transportable Existing Optical Ground Stations
Existing transportable optical ground stations are described in this section. These include the NASA/MIT Lincoln Laboratory's Lunar Lasercom Ground Terminal (LLGT), the Transportable Optical Ground Station (TOGS) by the German Aerospace Center (DLR), and the Transportable Adaptive Optical Ground Station (TAOGS) by Tesat Spacecom.
LLGT was the primary ground terminal developed for LLCD. A 622 Mb/s link was established between a lasercom terminal on the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft and LLGT. The ground station was designed by MIT Lincoln Laboratory and located at the White Sands Missile Range in New Mexico for operations. The ground station was designed to be transportable so that it could be developed near MIT Lincoln Laboratory and then transported to the White Sands location.
FIG. 8 shows the telescope assembly, which is contained in a clamshell dome and the support equipment is housed in a converted 40 ft shipping container. The telescope assembly includes four 15 cm uplink telescopes and four 40 cm downlink telescopes on an altazimuth mount. Having multiple apertures is a simple way of scaling to a large combined aperture and it provides spatial diversity to mitigate atmospheric effects. Behind each telescope are an FSM and InGaAs focal plane array to allow independent tracking of the downlink with 25 Hz closed-loop bandwidth.
The uplink includes four 10 W optical transmitters with adjustable divergence angles. The downlink telescopes couple to multi-mode polarization-maintaining fiber which transfer the photons to photon-counting superconducting nanowire arrays. These exotic detectors have demonstrated efficiencies of 0.5 photons/bit and enable the success of LLCD establishing high-rate links at lunar distances.
At the start of a link, LLGT and the space terminal point at each other based on known ephemeris data. After the space terminal sees the ground signal, it corrects its pointing to be seen by LLGT. Both sides of the link then close in on their targets and initiate communication. This process usually lasted only a few seconds.
DLR developed a transportable ground station to enable near real-time data transfer from Earth-observing satellites and aircraft. The system was first tested as part of DLR's VABENE project, in which a 1 Gb/s link was established between TOGS and a Dornier 228-212 aircraft. TOGS has also been used to track OPALS on the ISS, and is the primary ground station for the OSIRIS payload on the BiROS satellite.
FIGS. 9A and 9B show TOGS in its deployed and folded configurations, respectively. TOGS has a deployable 60 cm telescope in a Ritchey-Chrétien-Cassegrain configuration. The telescope is supported by an altazimuth mount on a structure with four adjustable legs. The structure has a mass of about 500 kg. The legs can level the mount and compensate for rough terrain. The telescope and mount fold into a truck which provides a means of transportation as well as an operations enclosure. To enable rapid calibration, dual-antenna GPS gives initial position and heading information. The mount also contains a tip/tilt sensor.
The optical assembly is mounted behind the telescope. A wide FOV camera is co-aligned with the main aperture to provide coarse feedback. Signal from the telescope is directed by a beam splitter onto a tracking camera and receiver to enable fine pointing. A movable lens allows focus adjustment. When the target is seen as a centroid on the tracking camera, a correction can be applied to the mount. Two 1550 nm, 5 W beacons are co-aligned with the receive telescope to illuminate the target.
Tesat Spacecom demonstrated coherent intersatellite laser communications in 2008 between two LEO satellites, and a modified version of the lasercom terminal was placed at ESA's Tenerife facility to demonstrate space-to-ground coherent communications. A 5.6 Gb/s bidirectional link was established between a 6.5 cm ground aperture and a 12.4 cm LEO aperture during campaigns in both Tenerife and Hawaii. Because the lasercom terminal was designed for intersatellite links using BPSK modulation, the ground terminal size had to be reduced to 6.5 cm and placed at high altitudes to overcome atmospheric effects.
Without adaptive optics, the ground station is limited in terms of its aperture size, which in turn increases the power required to establish a link. To overcome this, Tesat began developing TAOGS in partnership with Synopta and DLR. TAOGS has demonstrated 5.6 Gb/s communications with LEO satellites and 2.8 Gb/s uncoded to geostationary (GEO) satellites. TAOGS includes an optics container and an operations container as shown in FIG. 10.
TAOGS has a receive aperture of 27 cm and a separate transmit beam that can be adjusted between 2, 3.5, and 9.5 cm. There are two pointing assemblies, one of which contains the receive aperture and can transmit at 2 and 3.5 cm. A separate assembly is transmitter-only at 3.5 and 9.5 cm. Both assemblies can be seen in FIG. 10. A calibration procedure with star sightings is used to align the main pointing assembly up to an accuracy of 50 μrad.
TAOGS uses a CMOS camera behind the telescope as an acquisition sensor. An FSM is used to provide fine steering. A 96-element Shack-Hartmann sensor is used to detect the wavefront which is paired with a 12×12 actuator MEMS deformable mirror to provide wavefront correction. A separate mirror is used to implement point-ahead on the transmitted signal.
TABLE 1 (below) summarizes some parameters of the optical ground stations discussed above. While the development costs of the stations discussed are not published, they can be estimated from an Interagency Operations Advisory Optical Link Study Group with participants from Centre National d'Études Spatiales (CNES), DLR, ESA, JAXA, Korea Aerospace Research Institute (KARI), and NASA. The group estimated the cost of potential 40 cm ground stations in diverse locations intended for use with LEO satellites. Initial costs ranged between $1-5 million per site. For a mass estimate, consider TOGS, which has a mass on the order of 500 kg excluding the support equipment.
TABLE 1Summary of existing optical ground station parameters.Name/ReceiveMountf-Org.ApertureTyperatioNotesOGS/1mEnglishf/39Open-loop pointing ±50μ radESAequatorialFitted with adaptiveopticsOCTL/1mAltazimuthf/76Open-loop pointing ±17μ radJPLCo-aligned 20 cm f/7.5telescope for acquisitionLLGT/4 × 40cmAltazimuth—Deployable from 40-ftcontainerMITLLSuperconductingnanowire receiverfor lunar linksTOGS/60cmAltazimuthf/2.5Deployable from truckDLRCo-aligned wide FOVcamera foracquisitionTAOGS/27cmAltazimuth—Deployable fromcontainerTesatOpen-loop pointing ±200μ radFitted with adaptive optics